In the mid-level 40's I've been getting either Maraudon:Inner or Zul'Farrak as a random dungeon. I think ZF is slightly lower level, but it has the potential to be more difficult because of the large numbers of mobs and more complex boss encounters.
Zul'Farrak is a temple complex located in the Tanaris desert. It's full of trolls, witch doctors and zombies, and also lots of scarabs. The trolls are mainly in groups of 3 or 4, with some casters which need to be pulled back. This is because the trolls run when they get low on health and have a chance to pull other groups or wandering patrols. The trolls drop healing totems, which I could easily take out with a single hit from my wand, and they also cast hex, even on the tank, which should be dispelled fast.
The basilisk boss, hidden in a cave in the hills, can be a little difficult, as he spawns mobs behind you, does stuns and drops a rooting totem. My favourite encounter though is the pyramid escape, where you kill the executioner, loot his key then free the prisioners in cages awaiting sacrifice. You then need to survive wave after wave of zombie trolls, keeping the prisioners alive, and finally make it to the bottom of the pyramid and face a priest boss. You can then talk to the prisoners, who instead of thanking you will turn on you and you need to fight them. The hardest part of this whole encounter is for the tank to pick up all the mobs. Its helps to have a paladin of course. However, with higher level groups I found that we could just aoe everything down faster than it could kill us. Also, holy nova is really good here, allowing me to add a little damage and keep the whole party topped up without worrying about pulling aggro. The other bosses in this instance are pretty much tank and spank.
I've had some good fun in this place - it gives lots of exp and you can skip some bosses if you want a fast run. I've also met some very good groups...